Ocean acidification has been called the 'evil twin of warming'. While greenhouse gas emissions are increasing carbon dioxide levels in the atmosphere, resulting in a warmer climate, levels of carbon dioxide are also increasing in the ocean, causing pH to drop and the ocean to become more acidic. Around a quarter of the carbon dioxide emitted has so far been absorbed by the oceans.

“In the world's major oceans, pH has fallen by an average of 0.1 units since the start of industrialisation”, says Erik Gustafsson, oceanographer and researcher at Stockholm University's Baltic Sea Centre. “This may sound small, but it represents a change of more than 30 per cent.”

Researcher Erik Gustafsson. Photo: Lisa Bergqvist

Even though the term 'ocean acidification' is widely used, the ocean is still slightly alkaline (around pH 8.1), but the change is enough to affect many marine organisms, especially those that form calcium carbonate shells such as mussels and corals.

In coastal seas, such as the Baltic Sea, long-term changes of pH are more difficult to discern because pH is, in addition to the atmospheric carbon dioxide level, simultaneously affected by several other processes. Runoff from land is one of the important factors; river water can both raise and lower the pH depending on the rock types in the catchment area, and the inflow of seawater through the Danish straits also varies. In addition, seasonal variations in the sea have a major impact. Microalgae and cyanobacteria absorb carbon dioxide as they grow during spring and summer, causing carbon dioxide levels in the water to decrease and pH to rise. As the organic matter decomposes during the winter, carbon dioxide levels again rise and pH decreases.

“These oscillations are very strong. During the spring bloom in the central Baltic Sea, pH can increase by 0.5 units in a few weeks – much more than the long-term changes”, says Erik Gustafsson. “Overall, over time, we can see that ocean acidification has been counteracted by other processes in the northern and central parts of the Baltic Sea, but there are also studies from Danish estuaries that show that acidification there has been more severe than in the oceans.”

Significant changes in the long term

In a new study, Erik Gustafsson and his colleagues have used model simulations to investigate how the partial pressure of carbon dioxide (pCO2) and pH in the Baltic Sea may develop in the future under two of the different climate scenarios developed by the International Panel on Climate Change (IPCC): RCP 4.5 (when carbon dioxide emissions increase until 2040) and what is considered the worst possible scenario: RCP 8.5 (continued sharply increasing emissions).

The results show large differences in pCO2 and pH. Around the year 2100, pCO2 in the surface water around Gotland is projected to increase to an average of 550 μatm (microatmosphere) in the intermediate scenario RCP 4.5 (from today's approximately 420 μatm) and to almost 950 μatm in the worst climate scenario RCP 8.5. pH is projected to drop to about 7.9 from today's 8 under RCP 4.5. Under RCP 8.5, acidification accelerates around the year 2050 and pH reaches 7.7 in 2100.

“It is clear that in the long term, ocean acidification will be significant also in the Baltic Sea, especially if carbon dioxide emissions continue to increase according to RCP 8.5”, says Erik Gustafsson.

The development of pH in the Baltic Sea is mainly a consequence of the carbon dioxide emissions, but the inputs of nutrients is of some importance. Figure from the article "Causes and consequences of acidification in the Baltic Sea: implications for monitoring and management" publicerad i Scientific Reports.

Impact of eutrophication

The researchers have also examined how acidification may be affected by eutrophication, by combining the climate scenarios with two different scenarios for nutrient loads to the Baltic Sea. In the first case, the effect was investigated if the total input of nitrogen and phosphorus had continued at the high levels of the 1980s (1281 kton of nitrogen and 66 kton of phosphorus per year). In the second case, the input was reduced to the levels calculated to be required to achieve the objectives of the HELCOM Baltic Sea Action Plan for a sea unaffected by eutrophication (a total of 766 kton of nitrogen and 20 kton of phosphorus per year).

“Overall, acidification decreases slightly if the supply of nutrients is high, compared to if it is low," says Erik Gustafsson. "But it is about 0.02 pH units on average in both climate scenarios. How carbon dioxide emissions develop is therefore much more important.”

However, seasonal variations in pH become more pronounced when the nutrient supply is high.
“With more nutrients in the water, more algae are formed that assimilate carbon dioxide, and then carbon dioxide is again released as organic matter decomposes. So, the very lowest pH levels also occur with high nutrient supply,” says Erik Gustafsson.

The importance of monitoring

The organisms that live in the Baltic Sea are generally used to and adapted to the naturally large variations in pH. This could make it easier for them adapt also to long-term changes, compared with species in the open oceans. Studies on specific Baltic Sea species show that plankton can generally cope with higher carbon dioxide levels in the water, although there are indications that the species composition may change.

One species that has been shown in experiments to have more difficulty coping with acidification is the Baltic tellin (Macoma balthica), while the blue mussel (Mytilus edulis) and the ocean quahog (Arctica islandica) appear to be more tolerant. Cod may be adversely affected by acidification, although there are also studies suggesting the opposite, and herring larvae may also be affected. Possible changes to the food web may also affect fish populations.

Baltic tellin Photo: Ecomare/Sytske Dijksen /Wikimedia Commons.

“There are major uncertainties about how species are affected by acidification, but even more so about how they are affected by acidification in combination with other pressures such as warming and eutrophication”, says Erik Gustafsson.

In order to monitor the future development of acidification, it would be important to develop a system for regular sampling in different parts of the sea, of both pH and other related parameters such as the levels of dissolved inorganic carbon and alkalinity (the water's buffering capacity against acidification) as well as pCO2, says Erik Gustafsson. Such measurements are so far limited to a few stations and countries.

“More sampling is important to improve our understanding of how different organisms are affected by acidification. In addition to monitoring acidification trends, such measurements also provide information on primary production directly based on carbon cycling, which is ultimately also valuable for understanding the link between algal blooms and oxygen consumption in deep waters and for better monitoring eutrophication.”

Text: Lisa Bergqvist

Further reading

Gustafsson, E et al: Causes and consequences of acidification in the Baltic Sea: implications for monitoring and management

OMAI – Assessing acidification in the Baltic Sea, monitoring and scientific basis

Policy brief: Emerging ocean acidification threatens Baltic Sea ecosystems